1. Field of the Invention
The present invention relates generally to a method for producing highly conductive battery electrodes. The present invention also relates to a battery electrode produced by the process of the invention, an electrochemical cell or battery incorporating this battery electrode, and an implantable medical device, such as an implantable cardioverter defibrillator (ICD), incorporating an electrochemical cell having this battery electrode.
2. Related Art
In an effort to make a smaller, thinner implantable cardioverter defibrillator (ICD), considerable efforts have been made to miniaturize the electronic components of a device. The chip sets for operating the device have shrunk considerably, as have the high voltage capacitors, due to an increase of energy density by a factor of 2 to 3. The battery for an ICD, on the other hand, still utilizes very similar technology to what has been available over the last 20 years with very slight incremental improvements over time.
Electrochemical cells or batteries are used as the power source in many applications, including implantable medical devices. These electrochemical cells are designed for high current pulse discharge and low or no voltage delay. This design requirement is particularly important in an implantable cardioverter defibrillator (ICD), also referred to as an implantable defibrillator, since an ICD must deliver high voltage shocks to the heart immediately after the detection of arrhythmia. The cells must also have a high energy density to allow for the small size of implantable medical devices.
Typically, such an electrochemical cell or battery comprises a casing housing a positive electrode, a negative electrode, a separator between the positive and negative electrodes, and a conductive electrolyte. The casing of the electrochemical cell is typically of metal, preferably stainless steel. The positive electrode is the cathode during discharge and the negative electrode is the anode during discharge. The cathode is usually a solid having a body of active material comprising carbon fluoride, a transition metal oxide such as silver vanadium oxide (SVO), or other suitable materials known in the art. The cathode active material typically has a high energy density. The anode may comprise an alkali metal such as lithium or compounds and alloys thereof.
U.S. Pat. No. 4,830,940 to Keister et al. discloses a non-aqueous lithium battery with a solid cathode which includes silver vanadium oxide (SVO) as the active material. SVO has high volumetric capacity and high rate capability. SVO shows good current pulsing behavior at various levels of discharge and has a sloping discharge curve in a lithium cell that enables the prediction of end of life of the battery. SVO is also a semiconductor which allows cathodes to require less conductive material, such as carbon, to be added, thereby resulting in higher volumetric energy density. Thus, to increase the volumetric energy density of the cell, it is desirable to have a high percentage of SVO active material in the cathode and a low percentage of conductive material.
One of the limitations of the present silver vanadium oxide technology is in the fabrication of electrodes. The silver vanadium oxide is a ceramic powder that decomposes to other phases rather than reversibly melting, so it is not possible to melt cast the material onto a current collector for electrode fabrication. It is also difficult to sinter the SVO adequately due to the low melting/decomposition temperature. Furthermore, particularly at the early stages of discharge, the SVO is not particularly conductive, which would not allow for good migration of electrons from the current collector to the periphery of the electrode.
In order to overcome this last limitation, as well as the difficulty in sintering the material, a mixture of conductive additives and binders are generally employed to fabricate the cathode into a manufacturable form. U.S. Pat. No. 6,566,007 to Takeuchi et al. describes using acetylene black, carbon black, and/or graphite, as well as metal powders as the conducting additive, and using polymers such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF) in powder form as a binder material. In U.S. Pat. No. 5,558,680 to Takeuchi et al., an exemplary claim indicates that the mixture of active cathode material should be 94% by weight, mixed with 3% by weight binder, and 3% by weight conductive additive. However, the volume fraction is impacted significantly as SVO is considerably denser than either binder or the carbonaceous conductive additive.
Another preferred cathode active material comprises fluorinated carbon represented by the formula CFx, where x is typically between about 0.1 to about 1.9, preferably between about 0.5 to about 1.2. These active materials typically comprise carbon and fluorine, and include graphitic and non-graphitic forms of carbon, such as coke, charcoal, or activated carbon. Fluorinated carbons are particularly preferred cathode active materials in cells intended to be discharged under a light load for extended periods of time, such as for routine monitoring of cardiac functions by an implantable cardiac defibrillator. Like SVO, fluorinated carbons are capable of high volumetric energy density, thereby allowing for small sized implantable medical devices.
For low voltage batteries appropriate for pacemakers utilizing CFx technology, there is also need for binders and conductive additives. The binders and additives are highly similar as those for SVO batteries, and in U.S. Pat. No. 6,451,483 to Probst et al., with 91% active material and 5% conductive additives and 4% PTFE binder. While this is not as high a volume fraction as for SVO batteries, there is still room for improvement with the addition of greater amounts of active material.
Accordingly, what is needed is a method for fabricating silver vanadium oxide and fluorinated carbon electrodes having higher concentrations of active material.